Our washing machine is located in the basement. The drain feeds up to the main drain, about 5 feet off the floor. The rubber drain hose is looped up, down, and then up again to the drain. It's worked fine for years but occasionally there are several inches of water in the washer, even though the water supply is shut off and the machine has been fully drained for days. Of course other drains....kitchen and bathroom sinks, bathtub.....tie into the main drain as well but these are all free flowing. Is there a check-valve somewhere on the washers drain line that may be malfunctioning?
Thanks